Fig. 6: Loop movements and domain flexibility of tapasin.

Four X-ray determined structures (3F8U, 7TUE, 7TUF, 7TUG) and the AlphaFold model (AF-015533) were superposed. a–d illustrate the β1 and β2 strands of tapasin (Trp8 to Leu26) lying atop the HLA-B44 α2-1 and α2 helices (a, b, c, and d). a side view. b top view. c illustrates β1 and β2 (Trp8 to Leu26) from the tapasin–PaSta1(7TUF) complex. d AlphaFold model. As shown in the sequence alignment, only the tapasin structure from the PaSta1 complex (7TUF) and the AlphaFold computational model reveal a complete backbone through this region. The β1 and β2 strands clearly restrain (via Y84, T80, I142, and K146) the loop from descending into the B44:05 F pocket, and instead the loop provides a lid-like function. e shows the superposition of structures from Glu72 to Leu104 of tapasin for 7TUE, 5OPI (TAPBPR-MHC), 5WER (TAPBPR-MHC) and AF models of TAPBPR and tapasin. f superposition of β9 (Gly183-Arg188) and β10 (Leu196-Ala200) strands, and the hairpin loop (Gln189-His195) of the same structures. g, h, and i illustrate displacement of tapasin IgC, B44 α3, and β₂m movement respectively. For comparisons shown in h and i, α1 residues 54-84 of unliganded B44:05 (7TUD) were superposed onto the same residues of tapasin–B44:05-T73C (7TUE) and displacement of α3 (h) and of β₂m (i) was measured.